The insulin-like growth factor-I receptor (IGF-IR) mediates the growth and differentiation effects of the IGFs. Cancer cells overexpress IGF-IRs and this may impart to the cancer cells a growth potential. Recombinant human IGF-I (rhIGF-I)was administered, at varying doses, to nude mice that develop fibrosarcomas following injection of NIH-3T3 cells overexpressing IGF-I receptors. The administration of rhIGF-I at higher doses resulted in a shortened latency period, i.e., tumors appeared earlier than in vehicle-treated animals, and once the tumors appeared, they grew faster in the mice receiving the greater doses of rhIGF-I. The effect was also dependent on the level of IGF-IR expression, namely, the higher expression level of the receptor, the more responsive were the tumors. In studying the signaling pathways involved in IGF-I receptor- related tumor growth, we have identified a family of adapter proteins that may be involved in this process. CrkII and CrkL are SH2-SH3 domain-containing proteins. IGF-I receptor activation induces high-level tyrosine phosphorylation of both proteins and they both interact with insulin receptor substrate (IRS) molecules, whereas only CrkII interacts with the IGF-I receptor. Furthermore, the interaction of CrkII and CrkL with IRS-4 is different; Crk II interacts primarily via its SH2 domain, whereas CrkL requires both the SH2 domain and the N- terminal SH3 domain. This interaction with IRS-4 is tyrosine phosphorylation-dependent and deletion mutants demonstrated a central group of four residues (#700, 717, 743, 779) in IRS-4 that are involved in this interaction. When a fusion protein involving green fluorescent protein (GFP) and Crk proteins were expressed in cells in culture, followed by IGF-IR activation, GFP-CrkII was concentrated in the focal adhesion sites, whereas GFPCrkL remained diffusely expressed throughout the cell. Thus, CrkII may play a role in the differentiated function following IGF-IR activation, whereas CrkL mediates a more oncogenic response to IGF-IR activation. Their differential interaction with the IGF-IR and IRS molecules, and presumably downstream signaling molecules may be one of the mechanisms whereby they mediate these different functions.